A liquid crystal display device is a display device which is used for a notebook computer, a personal computer, a smart phone, or a television and a characteristic thereof is improved every year in accordance with enlargement of a demand of the liquid crystal display device.
A liquid crystal panel of a liquid crystal display device which is a non-emitting element requires a backlight unit due to its structure. The backlight unit is configured by various optical systems. Further, the backlight unit uses optical films which are periodically arranged in order to improve brightness.
FIG. 1 is a view schematically illustrating a structure of a liquid crystal display device of the related art.
As illustrated in FIG. 1, a backlight unit 10 includes a light emitting source 1, a reflecting plate 2, a light guide plate 3, a diffuser sheet 4, a first optical sheet 5, a second optical sheet 6, and a protecting sheet 7.
The light emitting source 1 is an element which generates visible ray and as for the light emitting source 1, a light emitting diode (LED) and a cold cathode fluorescent lamp (CCFL) may be selectively used.
The light emitted from the light emitting source 1 is incident onto the light guide plate 3 to be progressed while being totally reflected inside the light guide plate 3 and light which is incident onto a surface in the light guide plate 3 at an incident angle which is smaller than a threshold angle is not totally reflected but transmitted to be emitted to an upper side and a lower side.
In this case, the reflecting plate 2 reflects the light which is emitted to the lower side to be re-incident onto the light guide plate 3 to improve optical efficiency.
The diffuser sheet 4 diffuses the light emitted through an upper surface of the light guide plate 3 to uniformize brightness and broaden a viewing angle, so that the light which passes through the diffuser sheet 4 has reduced front emission brightness.
The first optical sheet 5 is configured by a base member 5b and a structural pattern 5a to primarily concentrate and emit light which enters from the diffuser sheet 4 so as to be refracted and vertically incident thereonto.
Further, the structural pattern 5a is integrally formed on an upper surface of the base member 5b to vertically refract and emit light which is incident through the base member 5b. 
The structural pattern 5a is generally formed to have a triangular shape and a vertical angle of the triangular shape is generally approximately 90 degrees.
The second optical sheet 6 has the same shape as the first optical sheet 5 and secondarily concentrates and emits the light which is primarily concentrated by the first optical sheet 5 in order to increase brightness thereof.
Here, the first optical sheet 5 and the second optical sheet 6 are integrally bonded such that an extending direction of the structural pattern of the first optical sheet 5 and an extending direction of the structural pattern of the second optical sheet 6 perpendicularly intersect, thereby increasing the brightness.
The protecting sheet 7 is attached onto an upper surface so as to prevent surface damage to the second optical sheet 6.
In the liquid crystal display device of the related art configured as described above, when the first optical sheet 5 and the second optical sheet 6 are bonded, the sheets generally pass through a pair of rollers and then are bonded by applying a pressure thereto.
However, when the first optical sheet 5 and the second optical sheet 6 are bonded by the method, since the first optical sheet 5 is pressurized in a direction of the second optical sheet 6, the first optical sheet 5 is bent due to pressure.